1. Field of the Invention
The present invention relates to an image processing method to output high quality half tone images in a device where dot output is unstable, and an image processor and storage medium thereof, and more particularly to an image processing method for performing high image quality processing using the error diffusion method, and an image processor and storage medium thereof.
2. Description of the Related Art
Recently half tone representation is demanded for a printer that outputs images by ON/OFF dots. Standard printers now can be classified into an inkjet type and an electro-photographic type, and algorithms for creating images are different depending on the printer type. In an inkjet printer, half tone images are generally created by the error diffusion method, and in an electro-photographic printer, half tone images are mainly created by the systematic dither method.
The error diffusion method uses an algorithm to obtain half tone images that maintain tone representation by diffusing a quantization error, which is generated when an input image is quantized by the number of tones possible by the output device, into peripheral pixels. Since this is serial processing in one pixel units, the output image excels in resolution, tone reproducibility and sharpness of the edges compared with other screening algorithms. Also dots are output at random, so moire is not generated. In the case of the systematic dither method, on the other hand, processing time is faster since only a comparison of the dither (threshold) matrix and input pixels is sufficient, but the quality of output images is not as good as the error diffusion method.
Unlike a system which ejects ink on paper, such as an inkjet type, the electro-photographic type creates a latent image by electrostatic force before attaching toner. Therefore the processing capability to perform electric control greatly influences the quality of images. It has been said that the error diffusion method, which can generate high quality output images, is not suitable for electro-photographic type half tone processing. The reason will be described with reference to FIG. 12 to FIG. 14.
FIG. 12 is a conceptual diagram of dot reproduction of an electro-photograph, FIG. 13 is a diagram depicting dot creation at a highlight part of the image, and FIG. 14 is a diagram depicting dot creation at a shadow part of the image. In FIG. 12, dot reproduction at 600 dpi and 300 dpi are compared as an example in the electro-photographic system. Here, the electro-photographic printer 100 in FIG. 12 can easily reproduce dots at 300 dpi, since one pixel can be created by a plurality of dots, a large latent image can be created, and electric force is strong. In other words, mid-tone representation is possible, and smooth images can be output.
A pixel size is halved at double resolution, 600 dpi. For example, as FIG. 13 shows, an isolated dot at 600 dpi is about 60 μm in size (=dot pitch 42.3 μm×root2). However, a dot diameter where toner adheres stably is about 4 times the drum film thickness. In a general electronic photograph, the drum film thickness is 20–30 μm, so a dot diameter where toner adheres stably is 80 μm. Therefore about 60 μm of dots will be unstable in size. The dots can be reproduced if the dots are next to each other, but an isolated dot where there are no nearby dots may not be reproduced.
Particularly in the error diffusion method where a quantization error is dispersed into peripheral pixels, isolated dots tend to be generated. Therefore, at 600 dpi, highlight tends to be reproduced at a density that is thinner than the desired density, and when dots increase, density suddenly increases and a high contrast image is generated, as FIG. 12 shows. In other words, mid-tone representation is difficult.
As FIG. 14 shows, in the case of the error diffusion method, dot distribution becomes checkers when a 50% area percentage is reproduced. Normally the dot diameter is set to root2 times of the dot pitch so that a diagonal line becomes continuous, so the checkers become almost solid (all black) when printed. Also the position of toner shifts by a process factor, such as toner-transfer process, and the white area where toner does not exist becomes narrower. Therefore density saturates when the area percentage is about 50%.
This error diffusion method was developed when standard printers were 300 dpi, where there were no problems since isolated dots are reproduced at 300 dpi, however, as resolution increases, a “jump” in the highlight part and the reproduction of the shadow part become problems. For these reasons, in a system where the stable creation of latent images becomes difficult as resolution increases, such as the case of an electro-photograph, half tone processing based on the error diffusion method has been inappropriate, and the dither method, where dot output can be controlled at will, has been normally used.
As a method to compensate dot reproduction when resolution increases, the following three methods have been proposed.
(1) A method of spreading isolated dots; As FIG. 15 shows, isolated dots are spread at output to prevent a tone jump in the highlight part (for example, Japanese Patent Application Laid-Open No. H10-19697).
(2) A method of switching screening between a shadow to mid-tone and a mid-tone to highlight; As FIG. 16 shows, quantization processing by line screen is performed from the shadow part to mid-tone, and dots are spread and output by gradually dropping the dot cycle to ½ from the mid-tone to highlight part (for example, Japanese Patent Application Laid-Open H8-156329).
(3) Error diffusion method considering dot gain;
This is an improved error diffusion method, including a prediction formula of dot gain in the algorithm of error diffusion, to reproduce a more accurate density and to prevent darkening in the shadow part (for example, U.S Pat. No. 5,087,981).
With the above conventional methods, however, the following problems occur.
(1) In the case of the first conventional method, an area where dots are spread and an area where dots are not spread coexist since the area is in proportion to the tone level, as shown in FIG. 17, so as a side effect of spreading isolated dots, tone reproduction may be reversed in a photograph. In other words, isolated dots can be reproduced correctly by spreading, but a density reversal occurs between one dot, which was spread, and four dots, which were not spread, as shown in FIG. 15 and FIG. 17, for example.
(2) In the case of the second conventional method, tone can be reproduced very well by switching the screens. However, in characters with low density, details, such as edges, become unclear, and the density maintenance of an output image and the clarity of edges become problems because the screen is rough, as shown in FIG. 18A, compared with the output images by the conventional error diffusion method shown in FIG. 18B.
(3) In the case of the third conventional method, density is reproduced, including the prediction formula (degree) of dot gain in the algorithm of error diffusion, so as to prevent dot gain generation and darkening in the shadow part, as shown in FIG. 19. Since the algorithm considers preventing an increase of density due to dot gain, darkening in the shadow part can be prevented by this method, but the probability of isolated dots appearing in the highlight part increases, which makes it difficult to generate dots in the highlight part.